Process for the production and recovery of ortho-xylene para-xylene and ethylbenzene

ABSTRACT

A DUAL ABSORPTION AND ISOMERIZATION PROCESS EMPLOYING A COMBINATION OF A FIRST ABSORPTION ZONE IN COMMUNICATION WITH A SECOND ABSORPTION AND AN ISOMERIZATION ZONE. THE PROCESS IS SUITABLE FOR THE SEPARATION OF VARIOUS C8 ISOMERS INTO INDIVIDUAL RELATIVELY PURE STREAMS CONTAINING THE INDIVIDUAL ISOMERS. A FIRST ADSORPTION ZONE SEPARATES PARA-XYLENE AND ETHYLBENZENE FROM THE OTHER C8 AROMATIC ISOMERS FED TO THAT ADSORPTION ZONE AND PASSES THE PARA-XYLENE AND ETHYLBENZENE TO A SECOND ADSORPTION ZONE WHEREIN PARA-XYLENE AND ETHYLBENZENE AR SEPARATED INTO RELATIVELY PURIFIED PARA-XYLENE AND ETHYLBENZENE STREAMS. THE REMAINING C8 AROUMATICS SEPARATED FROM THE PARA-XYLENE AND ETHYLBENZENE IN THE FIRST ADSORPTION ZONE ARE PASSED INTO AN ISOMERIZATION ZONE TO EFFECT THE PRODUCTION OF ADDITIONAL PARA-XYLENE WHICH IS EVENTUALLY RECYCLED TO THE FIRST ABSORPTION ZONE ALLOWING INCREASED YIELD OF PARA-XLENE BASED ON THE C8 AROMATIC FED TO THE FIRST ADSORPTION ZONE. AN ORTHO-XYLENE STREAM IS RECOVERED FROM THE EFFLUENT STREAM FROM THE ISIMERIZATION REACTION ZONE.

Jan. 18, 1972 L. o. s'rms ETAL 3,686,121

PROCESS FOR THE PRODUCTION AND RECOVERY OF ORTHO-XYLENE, PARA'XYLENE AND EI'HYBENZENE Filed Nov. '7. 1969 Laurence 0. Stine Donald B. Broughfon BY" 024% a {Mam A T TORNEYS 3,636,121 PROCESS FOR THE PRODUCTION AND RE- COVERY F ORTHO-XYLENE, PARA-XYLENE AND ETHYLBENZENE Laurence 0. Stine, Western Springs, and Donald B. Brougbton, Evanston, lit, assignors to Universal Oil Company, Des Plaines, Ill.

Filed Nov. 7, 1969, Ser. No. 874,919 Hnt. Cl. C07c 7/12, 15/08 U.S. Cl. 260-674 SA 9 Claims ABSTRACT OF THE DISCLOSURE A dual adsorption and isomerization process employing a combination of a first adsorption zone in communication with a second adsorption and an isomerization zone. The process is suitable for the separation of various C isomers into individual relatively pure streams containing the individual isomers. A first adsorption zone separates para-xylene and ethylbenzene from the other C aromatic isomers fed to that adsorption zone and passes the para-xylene and ethylbenzene to a second adsorption zone wherein para-Xylene and ethylbenzene are separated into relatively purified para-xylene and ethylbenzene streams. The remaining C aromatics separated from the para-Xylene and ethylbenzene in the first adsorption zone are passed into an isomerization zone to effect the production of additional para-xylene which is eventually recycled to the first adsorption zone allowing increased yield of para-xylene based on the C aromatic fed to the first adsorption zone. An ortho-xylene stream is recovered from the effluent stream from the isomerization reaction zone.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a process for the selective adsorption of certain C aromatic isomers to produce individually concentrated streams of the individual C aromatic isomers while isomerizing a portion of the C aromatic stream to effect the additional production of a given C aromatic isomer. More specifically, this invention pertains to a combination process employing two adsorption zones and an isomerization zone to effectively separate a C aromatic feed stream into concentrated streams of para-xylene, ethylbenzene and ortho-xylene. The two adsorption zones effect the separation of para- Xylene from the ethylbenzene while the isomerization zone allows either the additional production of ethylbenzene or para-Xylene depending on the products desired.

DESCRIPTION OF THE PRIOR ART It is known in the art that certain crystalline aluminosilicates can be used to selectively separate one type or class of hydrocarbons from another. Specifically, the

3,536,121 Patented Jan. 18, 1972 separation of C aromatic isomers can be effected employing various types of crystalline aluminosilicates to selectively adsorb a given C aromatic isomer, thereby allowing recovery of a desired C aromatic isomer by using a selected adsorbent. However, it has not been recognized in the art that a combination of adsorption zones together with a particular type isomerization reaction zone can be utilized in a given combination to effectively produce streams containing individual C aromatic iso mers in relatively pure form. The particular inventive concept disclosed herein employs two adsorption zones which selectively separate certain C aromatics and which allow a non-retained component of a C aromatic stream to be isomerized in a particular manner to form additional valuable C aromatic products. Additionally, the combination process of this invention allows the individual adsorption zones to 'be operated in the specific manner which allows particular desorbents to be used in the individual adsorption zones which are extremely well suited for the individual separation being effected in the individual adsorption zones and allows the adjustment of isomerization conditions to effect additional production of a desired C aromatic isomer.

SUMMARY It is an object of the present invention to present a combination process for the production of enriched streams containing ortho-xylene, para-xylene and ethylbenzene. More particularly, it is an object of the present combination invention to allow the dual adsorptive separation operation to effect the separation of a portion of the C aromatic feed stream into individual isomer components while isomerizing other C aromatic products to provide desired quantities of para-Xylene, ethylbenzene and ortho-Xylene. These and other objects of the present invention will come to light in view of the following description of the invention.

The various adsorption zones employed in the combination process of this invention effect the separation of the various C aromatic isomers which comprise paraxylene, meta-xylene, \ortho-xylene and ethylbenzene. We have found that separation of the various C isomers can be effected through the use of a crystalline aluminosilicate faujasite structured adsorbent. Common faujasite structured materials which can effectively separate the xylene isomers are the natural and synthetically-prepared Type X and Type Y structured zeolites which contain selected cations at the exchangeable cationic sites within the zeolite crystalline structure.

Both the natural and synthetic aluminosilicates may be used as adsorbents in the present invention. The crystalline zeolitic aluminosilicates encompassed by the present invention for use as an adsorbent in either or both of the adsorbent zones includes aluminosilicate cage structures in which the alumina and silica tetrahedra are intimately connected with each other in an open three-dimensional crystalline network. The tetrahedra are cross-"linked by the sharing of oxygen atoms. The spaces between the tetrahedra are occupied by water molecules prior to dehydration. Subsequent partial or total dehydration results in crystals interlaced with channels of molecular dimensions. Thus, the crystalline aluminosilicates are often referred to as molecular sieves. In the hydrated form, the crystalline aluminosilicates may be represented by the general formula presented in Equation 1 below,

M O:Al O i WSlOg I (l) where M is a cation which balances the electrovalence of the tetrahedra, n represents the valence of the cation, w represents the moles of SiO;,, and y the moles of water. The zeolite Type X can be represented in terms of the mole ratios of oxides as represented in the following Equation 2 below,

where M represents at least one cation having the valence of not more than 3, n represents the valence of M, and y is any value up to about 9, depending upon the identity of M and the degree of hydration of the crystalline. Zeolite X is described in U.S. Pat. No. 2,882,244.

The Type Y zeolite may be represented in terms of the mole ratios of oxides as represented in the following Equation 3,

where w is greater than about 3 and y is any value up to about 9, where M is a cation which balances the electrovalence of a tetrahedra, and n represents the valence of the cation.

The exchangeable cationic sites for both the Type X and Type Y structured zeolite, in general, can be defined as represented in Equations 2 and 3 above as M. Cation exchange or basic exchange methods as generally known to those familiar with the field of crystalline aluminosilicate production are generally performed by contacting the zeolite with an aqueous solution of soluble salts of the cation or cations desired to be exchanged onto the sieve. The degree of cation exchange is allowed to take place before the sieves are removed from the aqueous solution, washed and then dried to a desired water content. It is contemplated that in cation exchange operations, one or more cations may be placed on the zeolite.

In this specification the terms Type X or Type Y structured zeolites or faujasite-structured mate-rials refer to those structures having a framework consisting of a tetrahedral arrangement of truncated octahedra, for example, the octahedra adjoined at the octahydral faces by hexagonal prisms. The centers of the truncated octahedra occupy the same relative positions as the carbon atoms in diamond. The void spaces located within the faujasite or Type X or Type Y structured zeolites generally consist of elliptical shaped cavities approximately 13 angstroms in length, entered by apertures of distorted chair-shaped 12 member rings which have a free diameter of about 8 angstroms. The gross impression of this structure is that of a densely packed structure of oxygen atoms which surrounds a relatively large interstitial void. In this specification, the faujasite or Type X or Type Y structured materials shall include both the natural occurring minerals and their synthetically-prepared counterparts containing selected cations and specifically includes the various forms of the Type X and Type Y zeolites.

The cations or pairs of cations or mixtures of pairs and single cations which can be used in the adsorption zones of the process of this invention include metals or cations selected from potassium, rubidium, cesium, lithium, sodium, potassium, copper, silver, nickel, manganese, cadmium, the Group IIA metals and the pairs selected from barium and potassium, beryllium and potassium, magnesium and potassium, rubidium and potassium, copper and potassium, cesium and potassium, rubidium and barium, cesium and barium, potassium and barium, copper and silver, zinc and silver, copper and cadmium, rubidium and strontium and copper and potassium. Various mixtures of the individual list of single metals or pairs thereof can be used on the adsorbents for use in either adsorption zone provided the particular metal or cation or combination of both which is present within the Type X or Type Y structured zeolite is selected so that the C isomer separation which is required takes place.

The isomerization zone employed in the combination process of this invention contains catalysts selected for their abilities to isomerize the ortho-xylene and metaxylene stream from the first adsorption zone into various quantities of para-xylene or ethylbenzene, or both. The ability to employ an isomerization zone which, when through process manipulation or catalyst substitution, or both, allows production of various quantities of paraxylene or ethylbenzene according to the varying needs of para-xylene, ethylbenzene and ortho-xylene.

Typical catalysts which are used in normal isomerizetion reaction zones include inorganic refractory oxides containing platinum group metals and generally also containing a quantity of a halide. Specific isomerization catalysts include platinum group metals on alumina which also contains various quantities of fluorine, chlorine or bromine. In instances where it is desired not to produce ethylbenezene in the isomerization zone, that is, to convert a portion of the ortho-xylene and meta-xylene passed through the isomerization zone to para-Xylene, to use a catalyst containing palladium and chlorine on alumina. Additional isomerization catalysts of this character include palladium and a halide on alumina which also contains a small quantity of a crystalline aluminosilicate. Other type isomerization catalysts which may be employed for ethylbenezene production at the expense of para-xylene and meta-xylene products are platinum and chlorine on alumina. In many instances, the isomerization catalysts contain two or more different species of halides present on the base of a catalyst to allow the varied reaction to take place. A particularly preferred catalyst to be used in the isomerization zone where it is desired to enhance para-xylene production and ortho-xylene production which requires that the isomerization reaction not produce ethylbenzene is a catalyst containing approximately 0.75 wt. percent palladium and approximately 0.9% chlorine on an alumina base which base contains approximately 3 to 5% of a crystalline aluminosilicate selected from the mordenite species. In cases where ethylbenzene production at the expense of the production of para-xylene and meta-xylene is desired, the isomerization catalyst can generally comprise about 0.375 wt. percent platinum and from about 1 to about 3 wt. percent fluorine and from about 0.1 to about 1.0 wt. percent chlorine.

The adsorpton zones operating conditions can include temperatures in the range of from about ambient to about 250 C. and pressures generally above atmospheric and in most instances within the range of from about atmospheric to about 1,500 p.s.i.g. In instances in which a desorption is effected by using a liquid desorbent, it is generally performed in liquid phase operations. In other instances in which vapor operations or combination liquid or vapor operations are effected, adsorption can be effected in the liquid phase and desorption in the vapor phase. Of course, during these type operations in which different phases effect the adsorption and desorption operations, there may be temperature differences during the adsorption cycle and the desorption cycle or pressure differences between the two adsorption and desorption cycles or both temperature and pressure dilferences for both adsportion and desorption cycles. In some instances, desorption can be effected by using a gaseous hydrocarbon or other material at elevated temperature and reduced pressure or either one individually to effect desorption of the preferentially retained aromatic hydrocarbons on the adsorbents in the individual adsorption zones.

The overall operations effected in either or both adsorption zones can be performed by typical swing-bed type operations or by employing the simulated movingbed countercurernt flow operations. It is well known to those skilled in the separation art that manifold systems can be incorporated, where swing-bed operations are to be used, to allow continuous production of extract and raffinate streams by manipulating the input and output streams to the individual adsorption beds to effect continuous adsorption and desorption steps. The simulated moving-bed countercurrent flow operations are typically performed through the use of a series of inlet and outlet lines connected to an elongated bed of adsorbent which lines are advanced in the direction of the general fluid flow through the fixed-bed by using a particular type of a rotating valve which allows the various operations to take effect simultaneously at different areas of the fixed-bed. When observing the operations from a certain location within the adsorption bed, adsorption and desorption cycles are seen in a repetitive manner and in a way which allows the eflicient production of the desired extract and rafflnate streams. The overall countercurrent fixedbed simulated moving-bed type operations are generally demonstrated in US. Pat. No. 2,985,589. The operations of the process of this invention, however, are similar in some ways to the operations disclosed in that in the reference patent.

The isomerization zone operating conditions are determined by a particular product distribution which is desired in the efiluent stream from that zone. Typically, isomerization reaction zone operation include temperatures within the range of from about 200 C. to about 450 C., pressures of from about atmospheric to about 100 amtospheres, liquid hourly space velocities based on fresh feed passed into the reaction zone of from about 0.1 to about and hydrogen to hydrocarbon feed molal ratios of from about 1 to about 20. Additionally, in some instances, to control or maintain catalytic activity, halogen addition to the feed stock to the isomerization reaction zone may be effected. The concentration of halogen and the feed stock can be from about 1 to about 1,000 ppm.

DETAILED DESCRIPTION OF THE DRAWING The attached drawing shows the combination process of this invention which includes two adsorption zones and an isomerization zone with related separation equipment. The flow scheme showed in the attached drawing is simplified and does not contain valves, pumps, compressors, heat-exchangers or other detailed process equipment. It is assumed that those well versed in the refinery art would logically be able to determine the proper equipment needed in order to effect operation of the disclosed combination process.

Adsorption zones 1 and 2 effect the separation of paraxylene and ethylbenzene from the other C aromatics and the eventual separation of the para-xylene from the ethylbenzene. isomerization zone 3 receives raffinate or extract material from adsorption zone 1 and effects, depending on the desired product distribution, either isomerization of meta-xylene and ortho-xylene to additional para-xylene or the conversion of the metaxylene and some of the ortho-xylene to ethylbenzene which eventually is recycled and recovered from the process. Line 10, through which the feed stock to the process of this invention passes, is connected to adsorption zone 1. Entering adsorption zone 1, the feed stock is mixed with a recycle stream which passes through line 11. The total feed including recycle material passes through line 10 into adsorption zone 1 where there is effected the separation of para-xylene and ethylbenzene from the other C aromatics present in the feed. For the purposes of a specific disclosure, we shall assume that the para-xylene and ethylbenzene are selectively retained by the adsorbent in the adsorbent in the adsorption zone 1. The extract stream from adsorption zone 1 contains para-xylene and ethylbenzene and additionally contains a quantity of desorbent because the paraxylene and ethylbenzene are removed from the adsorbent by a desorbent flush stream which is removed along with the para-xylene and ethylbenzene. The extract stream passes through line 15 into separation zone 5 wherein para-xylene and ethylbenzene are separated from the desorbent material which is returned to the adsorption zone 1 via line 13 to be re-used. The para-xylene and ethylbenzene removed from separation zone 5 pass through line 16 into adsorption zone 2. The material paSS- ing through line 16 contains para-xylene and ethylbenzene. The separation effected in adsorption zone 2 allows the separation of para-xylene from ethylbenzene. The adsorbent contained in adsorption zone 2 may be paraxylene selective or ethylbenzene selective because of the fact that only two C aromatic isomers are passed into that bed and an adsorbent having a selectivity for only one of the two is sufficient to properly separate paraxylene and ethylbenzene. For the purpose of a specific disclosure, but not necessarily a limiting statement, we shall assume that the material passing through line 23 is an extract stream containing para-xylene and a desorbent material. This limitation requires that the adsorbent be para-xylene selective and that the material passing through line 22 be a rafiinate stream comprising the less selectively retained component and desorbent material. Material passing through line 22 passes into separation means 6 which separates ethylbenzene which passes out of process through line 24 from desorbent which passes via line 25 to be re-used in adsoption zone 2. The extract stream passing through line 23 which contains para-xylene and desorbent passes into separation zone 7 wherein desorbent is separated from the product para-xylene. The paraxylene is recovered via line 26 while the desorbent ma terial is recycled via line 27 into line 25 to be re-used in the process. Not shown in the drawing for either adsorbent bed are auxiliary desorbent withdrawal or addi tion lines which in many cases may be needed where the desorbent must be replenished or withdrawn from either or both systems. The material passing out of adsorption zone 1 through line 12 is a raflinate material and contains, in addition to desorbent material, meta-xylene plus ortho-zylene and any other non-retained components present in the feed mixture which may include other aromatic hydrocarbons such as benzene or toluene or paraffinic or naphthenic type materials. These materials pass into separation zone 4 where the desorbent material is separated from the raffinate components. The desorbent passes to line 13 to be re-used in the process while the purified metaand ortho-xylene containing material passes through line 17 into isomerization zone 3 where the desired isomerization reactions take place.

The effluent material from isomerization zone 3 passes through line 18 into separation zone 8. The efiluent material passing through line 18 may vary in composition depending on the operations effecting isomerization and the catalyst used in the isomerization reaction zone. Spe cifically, the isomerization zone effluent material may contain appreciable quantities of ethylbenzene which is produced at the expense of meta-xylene and ortho-xylene or may contain increased quantities of para-xylene when the para-xylene is a desired product. The efifuent material leaving separation zone 8 is separated into a light hydrocarbon stream which comprises toluene, benzene and lighter hydrocarbons which pass out of the process through line 19 and the C aromatics and heavier materials which pass out of separation zone 8 via line 20 and into separation zone 9 which essentially effects the separation of ortho-xylene and materials boiling at a temperature equal to or higher than ortho-xylene which pass through line 21 from materials which boil at a temperature below that of ortho-xylene which pass through line 11 which may be recycled to the feed intake line 10. A portion of the recycled material may be recovered prior to passage into adsorption zone 1.

The essential flow as described above is necessary to effect proper operations of the combination process. However, separating zones located between and on various lines contained in the drawing may be separation means such as distillation columns of tray or packed design or crystallizers may be utilized.

The para-xylene, ethylbenzene and ortho-xylene streams which pass out of the process may be further processed or collected as individually enriched streams of the individually namely products depending on the use intended for each.

In adsorptive-separation processes, an important factor that is used to determine the ability of a particular adsorbent to separate components of a feed mixture is the selectivity of the adsorbent for one component when compared to another component. The selectivity (B) as used throughout this specification is defined as the ratio of two particular components of an adsorbed phase over the ratio of the same two components of the unadsorbed phase at equilibrium conditions and is generally expressed in equation form as shown in Equation 4 below,

where B represents the selectively, x and y represent the individual components being compared, a represents the material adsorbed or retained by the adsorbent and it represents the external phase or the material which is sitting in the interstitial voids located between individual particles of adsorbent. The selectivity is measured at what are referred to as equilibrium conditions, such conditions occurring when the feed which was passed over a bed of adsorbent did not change in composition after contacting the bed of adsorbent, or in other words, there was no net transfer of material occurring between the unadsorbed external phase and the adsorbed phase 'when the selectivity of the two selected components was measured.

As can be seen where the selectivity of the two components approaches unity, there is no preferential adsorption of either component by the adsorbent. As the absolute value of the selectivity becomes greater than unity, there is a preferential selectivity by the adsorbent of one component. When comparing selectivity of component at over component y, a selectivity larger than unity indicates preferential adsorption of component x within the adsorbent when compared to component y, while a selectivity of less than unity would indicate that component y is preferentially adsorbed when compared to component x.

In testing various adsorbents, the selectivity as defined previously in Equation 4 was determined using appara tus and procedures generally described below. The apparatus used to measure the selectivity of a particular adsorbent consisted of a chamber of approximately 40 cc. volume having inlet and outlet ports at opposite ends of the chamber. The chamber was contained withina temperature control heating means and, in addition, pressure control equipment was used to operate the chamber at a constant predetermined pressure. Attached to the outlet line connected to the outlet of the chamber was a gas chromatograph which was used to periodically analyze a portion of the efiiuent stream leaving the adsorbent chamduring adsorption and desorption operations. A feed mixture having a known composition was passed through the adsorbent chamber at a regulated pressure and temperature until the effiuent composition flowing from the adsorbent chamber remained at a constant composition in dicating that there was no net transfer between the adsorbed phase within the adsorbent and the unadsorbed or external phase surrounding the adsorbent particles. A second mixture containing a hydrocarbon which was able Selectivity Ba.

to desorb the previously adsorbed components of the feed from the adsorbent was then passed through the adsorbent chamber. The gas chromatographed was used to monitor the unadsorbed or external phase and the material desorbed from within the adsorbent. Knowning the composition of these two streams and their respective flow rates, a selectivity for various components in the feed stream could be determined.

The feed stream which was used to measure the selectivities of various adsorbents used in the various adsorption zones consisted of equal quantities (8 and /3 vol. percent each) of ethylbenzene, para-xylene and meta xylene mixed with 2,2,4-trimethylpentane rendering a feed mixture containing vol. percent parafiinic material and 25 vol. percent C aromatic isomer material. The C aromatic isomers were diluted with the paraflin material to facilitate ease of analyzing the adsorbed and unadsorbed phases for the selectivity determination. Orthoxylene was excluded, since its presence would have complicated the analytical procedures, although previous experience indicated that the ortho-xylene isomer behaves substantially the same as the meta-xylene isomer. The desorbent materiai consisted of 25 vol. percent toluene, 74 vol. percent 2,2,4-trimethylpentane and 1 vol. percent neohexane which was used as a tracer to determine desorbent breakthrough in the effiuent stream leaving the adsorbent chamber. The adsorption and desorption cycles were performed in the vapor phase at about 260 C. and slightly above atmospheric pressure.

The following examples and illustrative embodiments more specifically show the various adsorbents and catalysts which can be used in the various units for the process of this invention and are not to be construed as undue limitations on the claimed invention.

EXAMPLE I In this example, various adsorbents were tested using the testing procedure as described previously to determine the capacity and selectivity of the various adsorbents for the individual C aromatic isomers. The two basic starting materials used in the production of the various adsorbents tested in this example were the Type X or Type Y zeolites. The adsorbents as indicated in the following table contained various single cations of combination of two cations present within the zeolite and were essentially totally ion-exchanged, that is, most of the cationic sites within the Type X or Type Y zeolite contained the indicated cation. An arbitrary cationic exchange procedure is described below.

A chromatographic-type exchange colunm was used to perform the cation exchange. The total number of cation equivalents used to exchange the original sodium form of the Type X or Type Y zeolite was three times the total equivalents of sodium present in the zeolite. The volume of solution containing the cations was six times the volume of the Type X or Type Y starting material with the cation solution flow rate at approximately to ml. per hour passing through the zeolite. Whenever two cations were used to simultaneously exchange the sodium in the original Type X or Type Y zeolite, the total number of equivalents of each cation used was halved; all other conditions remained equal. After the cation exchange procedure had been completed, the adsorbent was waterwashed, air-equilibrated and finally treated with air in a multle furnace at about 550 C. After heating, the sieves Were cooled in an inert atmosphere containing no water and then used in the adsorbent chamber as previously described. The table below shows the capacity of the sieve for the various C aromatic isomers represented by the milliliters of the particular C aromatic isomer adsorbed per 40 ml. of sieve in the adsorbent chamber. Tht selectivities were determined using Equation 4 above and are represented for determining the selectivities of the combinations of paraxylene and ethylbenzene, para-xylene and meta-xylene, meta-xylene and ethylbenzene.

Capacity, ml./40 ml. sieve Selectivity, B Zeolite Catio11(s) Adsorbent type on sieve P M E E P/e P/m M/e Y Li 1. 74 2. 43 1. 14 5. 31 1. 52 0.72 2.13 Y Na 1.69 2. 23 1. 28 5. 20 1. 32 0.75 1. 74 Y K 2.05 1. 12 1. 76 4. 93 1.16 1.83 0. 64 Y Rb 1.84 1. 22 1. 92 4. 98 1. 51 0.96 0.64 Y Cs 1. 38 0.92 1. 73 4.03 1. 50 0.80 0. 53 Y Be 0.84 0.93 0.78 2. 55 1.08 0.91 1. 19 Y Mg 1. 55 2. 65 0.93 5.13 1. 67 0. 59 2. 85

Ga 1.01 2. 86 0.86 4. 73 1.17 0.35 3. 32 Y Sr 1. 12 2. 56 0. 80 4. 48 1. 40 0. 44 3. 20 Y Ba 1. 51 1. 19 0.82 3. 52 1.85 1. 27 l. 45 Y K-Ba 2. 70 0.72 1. 29 4. 71 2. 3. 76 0.56 Y K-Be 2. 24 1. 06 1. 55 4. 85 1. 44 2. 11 0.68 Y K-Mg 2. 16 0. 96 1. 54 4. 66 1. 41 2. 25 0. 62 Y K- Rb 1. 70 O. 94 1. 60 4. 24 1. 06 1. 80 0. 59 K-Cs 1. 72 0.97 1. 67 4. 37 1.03 1. 79 0. 58 RbBa 2. 03 0. 99 1. 44 4. 46 1. 41 2. 05 0. 69 05-13:) 1. 26 1. l8 1. 43 4. 47 1. 30 1. 57 0.82 Na 1. 50 1. 48 1. 30 4. 28 1. 1.02 1. 14 Ba-K 2. 47 0. 99 1. 22 4. 68 2. 03 2. 49 0. 81 Ag 1. 63 1. 53 1. l8 4. 34 1. 38 1. 07 1. 30 Mn 1. 14 1. 74 9.91 3. 79 1. 0.66 1.91 Cd 1. 13 1. 84 0.94 3. 91 1. 19 0.61 1. 96 Cu-Cd 1. 18 2. 18 0. 98 4. 34 1. 20 0. 54 2. 22 Cu-Ag 1. 67 2. 65 l. 06 5. 38 1. 58 O. 63 2. 50 Zn-Ag 2. 33 2. 63 1. 46 6. 42 1. 59 0.89 1.80 Cu 1. 33 2. 14 0. 91 4.32 1. 46 0.62 2. 36 Cu-K 1. 90 1. 15 1.60 4. 65 1. 19 1.65 0. 72

As can be seen from the table, the particular adsorbents which can be used in the first adsorption zone to separate meta-xylene and ortho-xylene from para-xylene and ethylbenzene include adsorbents A, B, F, G, H, I, U, V, W, X, Y and Z where the meta-xylene and ortho-xylene are selectivity adsorbed C aromatics, while sieves C, K, L, M, N, O, P, Q, S and AA can be used Where para-xylene and ethylbenzene are selectively retained by the sieve with the meta-xylene and ortho-xylene being the rafiinate materials. Either one of these sets or mixtures of these sets may be used in the first adsorption zone. The basic con sideration is Whether in the operation of this particular adsorption zone, it is required that ortho-xylene and metaxylene be either the extract material or the rafi'inate material with the adsorbent selection based on that determination.

In adsorption zone 2, all of the adsorbents shown in the table can be used and when any of the adsorbents from Table 1 is selected to be used in the second adsorption zone, para-xylene is the selectively adsorbed component.

As can be seen, the proper selection of adsorbents in the two adsorption zones will allow the first separation of para-xylene and ethylbenzene from a mixture containing para-xylene, ethylbenzene, ortho-xylene and meta-Xylene, while the second adsorbent bed affects the separation of para-xylene from ethylbenzene.

Various modes of operation can be effected in adsorption zone 1 by selecting a proper adsorbent from those listed in the table to effectively adsorb either para-xylene and ethylbenzene or ortho-xylene and meta-xylene. Depending on the adsorbent selection for adsorption zone 1 either a raffinate or extract stream is passed to adsorption Zone 2, but in all cases the material flowing to the second adsorption zone contains para-xylene and ethylbenzene. It has been found that in cases where a reformate feed is fed to the process that naphthenes and other materials comprising parafiinic type hydrocarbons are present with the aromatic material fed to the process. The non-aromatic materials, from previous experiments, have been found to not be selectively retained by the adsorbents used in this invention and end up as part of the rafiinate material. The non-aromatic materials in the raifinate are in most all cases slightly lower-boiling than the C aromatic streams and in many cases boil within the boiling range of benzene or toluene. It is for this reason that the preferred desorbent to be used in the first adsorption zone is a material boiling at a heavier temperature than the C aromatics. This allows the desorbent which is present in a raftinate stream to be easily fractionated from both the C aromatics present in the rafiinate and the non-aromatic materials also present. The non-aromatic materials are preferably separated and recovered for further processing as a relatively purified non-aromatic stream. In order to prevent the non-aromatic materials from being further passed on to adsorption zone 2, it is preferred to use an adsorbent in adsorption zone 1 which selectively retains para-xylene and ethylbenzene, thereby allowing the raflinate material to be passed, if desired, into the isomerization zone.

The preferred mode of operations of adsorption zone 2 require that material fed to this adsorption zone contain essentially non-aromatic materials. This requires that the first adsorption zone be operated in such a manner as to selectively adsorb para-xylene and ethylbenzene while allowing the non-aromatic portion of the material fed to that adsorption zone plus meta-xylene and ortho-xylene if present to be present in a raflinate stream.

It has been found in pilot plant scale testing apparatus that to effect an efficient separation of para-xylene from ethylbenzene that a toluene desorbent is preferred, the toluene is lower boiling than the C aromatic isomers and when the feed to the second adsorption zone is an extract material from the first adsorption zone, there are no non-aromatics present in that stream which can reduce the effectiveness of toluene desorbent properties. Also, the toluene can easily be separated from the C aromatics by distillation techniques when there are no non-aromatics in the extract and raffinate streams leaving the second adsorption zone. The preferred desorbent material, toluene, which is present in both the rafifinate and extract streams leaving adsorption zone 2 is easily separated from the para-xylene and ethylbenzene materials by distillation and because of the aforementioned limitation that non-aromatic materials be absent from feed stocks passed into this zone, there is no interference or contamination of the desorbent by non-aromatic materials forming close to its boiling range. This allows ethylbenzene and para-xylene to be recovered from adsorption zone 2 in a highly purified form.

The following illustrative embodiment present a specific method of operation describing the overall operations of the combination process of this invention.

ILLUSTRATION 1 A reformate effluent which contains a relatively high concentration of aromatic hydrocarbons plus some nonaromatic materials comprising paraffinic and naphthenic materials is passed into the first adsorption zone along with recycle material from an isomerization zone. The first adsorption zone is operated in a manner such that para-xylene and ethylbenzene are the selectively retained components of the feed stock passed into that zone. The recycle stream from the xylene isomerization zone as previously mentioned may vary in composition depending 11 upon the particular product distribution required of the overall process. The isomerization zone may effect the production of para-Xylene and meta-xylene at the eX pense of ethylbenzene and ortho-xylene or may produce para-xylene, meta-xylene and ethylbenzenes at the expense of ortho-xylene. The extract material from the first adsorption zone contains, in addition to para-xylene and ethylbenzene, a desorbent material preferably diethylbenzene. The diethylbenzene concentration in the extract stream which is passed into a separation means to separate desorbent material from the C aromatics is approximately from about to about 50% diethylbenzene. The purified para-xylene and ethylbenzene stream Withdrawn from this separation means contains essentially no desorbent material and is passed into a second adsorption zone which contains any one of the adsorbents selected from the above table and which is selective for the retention of para-xylene as compared to ethylbenzene. The extract stream removed from the second adsorption zone contains para-Xylene and as indicated, toluene, which is -0 used as a preferred desorbent. The extract stream is passed into a separation means which separates the para-xylene from the toluene which is re-used in the process. The raffinate stream which comprises ethylbenzene and toluene is withdrawn from the second adsorption zone and is passed into another separation means where ethylbenzene is recovered as a relatively purified product with the toluene being passed back into adsorption zone 2 to be reused as a desorbent material. The rafiinate material from adsorption zone 1 also contains diethylbenzene, which is the preferred desorbent for that zones operations and is passed into a separation means where ortho-xylene, metaxylene and any non-aromatics are essentially separated from the diethylbenzene, which is recycled for re-use as desorbent. The ortho-xylene, meta-xylene and non-aromatic hydrocarbons are passed into an isomerization zone where isomerziation takes place to the extent and in a manner dictated by the required product distribution. The efiiuent material from the isomerization zone passes into a separation means Where materials boiling at a lower temperature than the C aromatics, generally the non-aromatic hydrocarbons comprising cracked parafiins, and benzene and toluene either from products of side reaction or present originally in the feed passed into the first adsorbent bed, are separated from the C aromatics. The C aromatics are then further separated in another separation means with the lower boiling materials comprising meta-Xylene or para-xylene or ethylbenzene or various combinations of all being recycled back to the first adsorption zone. The material recycled to adsorption zone 1 has been previously mentioned and vary in compositions depending upon the particular product distribution required and can comprise either essentially para-xylene, essentially meta-xylene stream or may also contain ethylbenzene present along with various quantities of paraxylene or methyl-xylene. The higher boiling components comprise ortho-xylene and C aromatics and are withdrawn from the process as a product stream relatively pure in ortho-xylene.

As can be seen from the above illustration, the operat ing conditions in the isomerization zone effect production of various quantities of para-xylene or ethylbenzene. Also, the adsorptive separation zones can be operated in a manner by using ditferent adsorbents to produce, in a desired procedure, para-xylene, ethylbenzene and ortho-xylene rich streams. It is generally considered that, in this invention, the para-xylene and ethylbenzenes Which are present in the feed stock fed to the first adsorption zone are not passed into the isomerization zone but are recovered prior to the passage of hydrocarbons thereto.

PREFERRED EMBODIMENTS A broad embodiment of the combination process of this invention resides in a combination process wherein two separate adsorptive separation zones are employed to effectively separate relatively purified para-xylene and ethylbenzene streams and to allow the stream concentrated in meta-xylene or ortho-xylene to be passed into an isom erization zone to be isomerized in a manner in which a maximum desired product distribution is effected with a portion of the effluent stream from said isomerization zone recycled to the adsorptive separation zones to allow additional recovery of purified para-xylene and ethylbenzene products.

Another broad embodiment of the process of this invention resides in effecting isomerization of a stream containing meta-xylene, or ortho-Xylene, or both, in combination with two adsorptive separation zones to effective- 1y allow additional production of para-xylene or ethylbenzene or various additional quantities of both by the particular mode of operations effected in the isomerization zone and the choice of catalyst used therein.

We claim as our invention:

1. A process for the production and recovery of Cg aromatic isomers, which process comprises the steps of:

(A) Contacting a mixture containing ortho-xylene, meta-Xylene, para-xylene and ethylbenzene with a bed of a crystalline aluminosilicate adsorbent in a first adsorption zone at conditions to effect the selective retention of a para-xylene and ethylbenzene by said adsorbent;

(B) Passing at least a portion of the non-retained components of said mixture characterized as rafiinate material which contains meta-xylene and ortho-xylene to an isomerization reaction zone to effect the conversion of a portion of said non-retained components to at least one C isomer selected from the group consisting of ethylbenzene and para-xylene;

(C) Recovering ortho-xylene from an isomerization zone efiluent stream;

(D) Recycling a portion of said isomerization zone effluent stream to said first adsorption zone;

(E) Contacting at least a portion of the selectively retained para-xylene and ethylbenzene with a bed of a crystalline aluminosilicate adsorbent in a second adsorption zone at conditions to eflect the selective retention of one of said selectively retained para-xylene and ethylbenzene isomers; and

(F) Recovering from said second adsorption zone enriched para-xylene and ethylbenzene streams.

2. The process of claim 1 further characterized in that said crystalline aluminosilicate adsorbent in said first and second adsorption zones is selected from the group consisting of Type X and Type Y structured zeolites.

3. The process of claim 2 further characterized in that said crystalline aluminosilicate adsorbent in said first adsorption zone contains at least one metal or pair of metals selected from the group consisting of potassium, lithium, sodium, beryllium, magnesium, calcium, strontium, manganese, cadmium, copper, barium and potassium, beryllium and potassium, magnesium and potassium, rubidium and potassium, copper and potassium, rubidium and barium, cesium and barium, copper and silver, zinc and silver, and, copper and cadmium.

4. The process of claim 2 further characterized in that said crystalline aluminosilicate adsorbent in said second adsorption zone contains at least one metal or metal pair selected from the group consisting of the Group Il-A metals, the Group IA metals, copper, silver, manganese, cadmium, cesium and barium, rubidium and barium, p0- tassium and barium, copper and silver, zinc and silver, copper and cadmium, copper and potassium, potassium and beryllium, potassium and magnesium, potassium and rubidium, and, potassium and cesium.

5. The process of claim 1 further characterized in that said selective retention of para-xylene and ethylbenzene by the adsorbent in said first adsorption zone is effected by the steps of:

(A) Contacting said feed mixture with a bed of an adsorbent at conditions to eifect the selective reten- 13 tion of para-xylene and ethylbenzene by said adsorbent;

(B) Withdrawing from said bed of adsorbent a rafiinate stream made up of non-retained feed components containing ortho-xylene and meta-xylene;

(C) Contacting said bed of adsorbent with a stream containing a first desorbent to effect desorption of any meta-Xylene and ortho-xylene retained within the pores of the adsorbent and thereafter contacting said bed of adsorbent with an increased concentration of a first desorbent at conditions to effect the desorption of para-xylene and ethylbenzene from said adsorbent; and

(D) Recovering from said first adsorption zone a para-Xylene and ethylbenzene enriched stream.

6. The process of claim 1 further characterized in that said selectively retained para-xylene and ethylbenzene are separated into an enriched para-Xylene stream and an enriched ethylbenzene stream in said second adsorption zone by the steps of:

(A) Contacting said para-Xylene and ethylbenzene with a bed of a crystalline aluminosilicate adsorbent in a second adsorption zone at conditions to effect the selective retention of one of said para-xylene and ethylbenzene isomers by said adsorbent;

(B) Withdrawing from said bed of adsorbent a raffinate stream containing a non-retained isomer of said paraxylene and ethylbenzene isomers;

(C) Contacting said bed of adsorbent With a stream containing a second desorbent to effect desorption said retained isomer is para-xylene and said non-retained isomer is ethylbenzene.

8. The process of claim 5 further characterized in that said first desorbent comprises a diethylbenzene.

9. The process of claim 6 further characterized in that said second desorbent comprises toluene.

References Cited UNITED STATES PATENTS 3,078,318 2/1963 Berger 260668 3,114,782 12/1963 Fleck et a1 260674 3,177,264 4/1965 Buchsbaum et a1. 260674 3,126,425 3/1964 Ebcrly et al. 260674 3,524,895 8/1970 Chen et a1. 260-674 DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, Assistant Examiner US. Cl. X.R. 

